1. Field of the Invention
The present invention relates to the deposition of an aluminum coating on a metal part, especially on a hollow metal part comprising an internal liner. It more particularly targets the application of such a coating to hollow turbomachine vanes incorporating a liner for distributing coolant.
2. Discussion of the Background
A gas turbine engine, such as is used for propulsion in the aeronautical field, comprises an atmospheric air inlet which communicates with one or more compressors, generally including one fan, that are rotated about one and the same axis. The primary stream of this air, after having been compressed, supplies a combustion chamber arranged annularly around this axis and is mixed with a fuel to provide hot gases downstream to one or more turbines through which these gases are expanded, the turbine rotors driving the compression rotors. The engines operate at a temperature of the engine gases at the turbine inlet that is sought to be as high as possible because the performances are linked to it. For this purpose, the materials are chosen to withstand these operating conditions and the walls of parts swept past by the hot gases, such as the nozzles or the rotating turbine blades, are provided with cooling means. Furthermore, due to their metallic composition as superalloy based on nickel or cobalt, it is also necessary to protect them from erosion and corrosion caused by the constituents of the engine gases at these temperatures.
One known means for providing the protection of these parts is to deposit an aluminum-based coating on the surfaces susceptible to attack by the gases. The aluminum is attached to the substrate by metal interdiffusion and forms a protective oxide layer at the surface. The thickness of the coating is of the order of a few tens of microns.
The present invention relates to the technique, known per se, of depositing aluminum in the vapor phase, otherwise known as vapor phase aluminization. According to the process, the parts to be treated are placed in a chamber in which the atmosphere is composed of a mixture of an inert or reducing gas, for example argon or hydrogen, and an active gas comprising an aluminum halide. At the reaction temperature, between 900° C. and 1150° C., the halide decomposes at the surface of the part to gaseous halogen and aluminum which diffuses into the metal.
The halide is produced by placing, in the chamber with the parts to be treated, blocks of aluminum metal or of an aluminum alloy that form the donor, in the presence of granules of a halogen compound, a chlorine or fluorine compound, that form the activator. The inert gas is circulated over the activator at a temperature that allows the sublimation of the halogen which is entrained over the donor and with which it reacts to produce the metal hydride which, at this temperature, is in the vapor form. The halide then decomposes in contact with the metal substrate to be coated, allowing the deposition of aluminum; the gaseous halogen is reformed.
When the stator parts and also the rotor parts are provided with internal cavities through which a coolant and air withdrawn from the compressor travel, it was observed that the walls of these cavities were also subject to corrosion. Returns of parts used in engines operating in certain environments have shown an attack of these surfaces. The following have been found, for example, internal corrosion of the nozzles, releases of flakes of corrosion into the cavity of the nozzles, blocking of the trailing edge vent holes, etc. Protection for these types of parts is therefore also necessary.
The vapor phase aluminization method is particularly suitable a priori for applying a protective coating since the carrier gas and the active components are capable of penetrating into the narrow passages for circulation of the coolant insofar as these passages are open. Reality shows that this is not the case. The thickness of the protective layer is not uniform; it greatly decreases starting from the access orifices of the cavities. Furthermore, accumulations are formed at the vent holes of the cavities, reducing the flow area and the cooling properties of the part.
In Patent Application FR 2830874 in the name of the Applicant, a process is described for the vapor phase aluminization of metal turbomachine parts provided with holes and cavities that communicate with the outside, according to which a gaseous precursor of the deposit to be made comprising an aluminum compound is brought, by means of a carrier gas, into contact with the surfaces of the part placed in a chamber, the carrier gas is either helium or argon and the pressure in the chamber is chosen so that the mean free path of the carrier gas molecules is two times greater than that of the argon molecules under atmospheric pressure. The mean free path of the molecules is usually defined as the ratio 1/P×D2 where P is the pressure in the chamber and D is the molecular diameter.
By lengthening the mean free path of the carrier gas molecules, the diffusion of the halide in the internal channels is increased and the thickness of the deposit in the zones that are less accessible via the conventional method is increased; the overall protection thereof is improved. The increase in the free path results either from the choice of the carrier gas, here helium, or from a reduction in the pressure as can be deduced from the above formula.